An Inertial Navigation System (INS) is employed to a provide navigation solution to guide vehicles such as aircraft and may receive input from an Inertial Measurement Unit (IMU) to generate the navigation solution. The navigation solution includes position, velocity, and attitude for example for vehicle control. A global positioning system can also be employed to correct or calibrate the navigation solution in the INS. Inertial navigation systems usually can provide an accurate solution for a short period of time. For example, INS accelerometers in the IMU produce an unknown bias signal that appears as a genuine specific force. This may be integrated twice and produces an error in position. Additionally, the INS software uses an estimate of the angular position of the accelerometers when conducting this integration. Typically, the angular position is tracked through an integration of the angular rate from the gyroscopic sensors in the IMU. These also can produce unknown biases that affect the integration to generate the position of the unit. The GPS provides an absolute drift-free position value that can be used to reset the INS solution or can be blended with it by use of a mathematical algorithm, such as a Kalman Filter. The angular orientation of the unit can be inferred from the series of position updates from the GPS. The change in the error in position relative to the GPS can be used to estimate the unknown angle error.
The benefits of using GPS with an INS are that the INS may be calibrated by the GPS signals and that the INS can provide position and angle updates at a quicker rate than GPS. For high dynamic vehicles, such as missiles and aircraft, the INS fills in the gaps between GPS positions. Additionally, GPS may lose its signal and the INS can continue to compute the position and angle during the period of lost GPS signal. The two systems are complementary and are often employed together. Even with GPS corrections to the INS, the INS position estimates can still drift with time. In some cases, the drift can account for more than one hundred meters of error in a period of 10 minutes, for example.